The present invention relates to a photomask used for producing integrated circuits of high integration density, e.g., large-scale integrated circuits (LSI), very large-scale integrated circuits (VLSI), etc., and to a photomask blank used to produce the photomask. The present invention also relates to a method of producing the photomask blank. More particularly, the present invention relates to a halftone phase shift photomask whereby a projected image of very small size can be obtained, and also to a halftone phase shift photomask blank for producing the halftone phase shift photomask. Further, the present invention relates to a method of producing the halftone phase shift photomask blank.
Semiconductor integrated circuits, e.g., IC, LSI, VLSI, etc., are produced by repeating lithography processes using photomasks. To form semiconductor integrated circuits of very small size in particular, use of a phase shift photomask has heretofore been considered, as disclosed, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 58-173744 (1983) and Japanese Patent Application Post-Exam Publication No. 62-59296 (1987), and phase shift photomasks having various arrangements have been proposed. Among them, what is called haltone phase shift photomask such as is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 4-136854 (1992), U.S. Pat. No. 4,890,309, etc. has attracted attention from the expectation that it will soon be put to practical use, and some proposals have been made with regard to arrangements and materials of the halftone phase shift photomask, which enable an improvement in yield and a reduction in cost as a result of a reduction in the number of manufacturing steps required. For example, see Japanese Patent Application Laid-Open (KOKAI) Nos. 5-2259 (1993) and 5-127361 (1993).
The halftone phase shift photomask will briefly be explained below with reference to the accompanying drawings. FIGS. 16(a) to 16(d) show the principle of the halftone phase shift lithography, and FIGS. 17(a) to 17(d) show a conventional lithography method. FIGS. 16(a) and 17(a) are sectional views showing photomasks. FIGS. 16(b) and 17(b) each show the amplitude of light on the photomask. FIGS. 16(c) and 17(c) each show the amplitude of light on a wafer. FIGS. 16(d) and 17(d) each show the light intensity on the wafer. Reference numerals 101 and 201 denote substrates, and 202 a 100% light-blocking film. A semitransparent film 102 shifts the phase of incident light through substantially 180.degree. and has a transmittance of 1% to 50%. Reference numerals 103 and 203 denote incident light. In the conventional method, as shown in FIG. 17(a), the 100% light-blocking film 202, which is made of chromium, for example, is formed on the substrate 201, which is made of quartz (fused silica), for example, and the light-blocking film 202 is merely formed with a light-transmitting portion in a desired pattern. Accordingly, the light intensity distribution on the wafer has a gentle slope, as shown in FIG. 17(d). As a result, the resolution is degraded. In the halftone phase shift lithography, on the other hand, the light passing through the semitransparent film 102 and the light passing through the opening in the film 102 are in substantially inverse relation to each other in terms of phase. Accordingly, the light intensity at the pattern boundary portion on the wafer is 0, as shown in FIG. 16(d). Thus, it is possible to prevent the light intensity distribution from exhibiting a gentle slope. Accordingly, the resolution can be improved.
It should be noted here that a phase shift lithography process of a type other than the halftone phase shift lithography requires at least two photoengraving processes to produce a mask pattern because the light-blocking film and the phase shifter film have different patterns, whereas the halftone phase shift lithography essentially requires only one photoengraving process because it involves only one pattern; this is a great advantage of the halftone phase shift lithography.
Incidentally, the semitransparent film 102 of the halftone phase shift photomask is demanded to perform two functions, that is, phase inversion and transmittance control. Regarding the phase inversion function, the semitransparent film 102 should be arranged such that exposure light passing through the halftone phase shift portion and exposure light passing through the opening in the film 102 are in substantially inverse relation to each other in terms of phase. If the semitransparent film 102 is treated as an absorbing film which is shown in M. Born, E. Wolf "Principles of Optics", pp. 628-632, for example, multiple interference can be neglected. Accordingly, the phase change .phi. of perpendicularly transmitted light may be calculated as follows: ##EQU1## where .phi. is the phase change of light perpendicularly passing through a photomask having a multilayer (m-2 layers) film formed on a substrate, x.sup.k,k+l is the phase change occurring at the interface between the k-th layer and the (k+l)th layer, u.sub.k and d.sub.k are the refractive index of a material constituting the k-th layer and the thickness of the k-th layer, respectively, and .lambda. is the wavelength of exposure light. It is assumed here that the layer of k=1 is the substrate, and the layer of k=m is air.
The above-described phase shift effect is obtained when .phi. falls within the range of n.pi..+-..pi./3 radians (n is an odd integer).
Meanwhile, the transmittance for exposure light of the halftone phase shift portion that is most suitable for obtaining the halftone phase shift effect is determined by the size, area, layout, configuration, etc., of the transfer pattern. That is, the optimal transmittance varies with each particular transfer pattern. Practically, it is necessary in order to obtain the halftone phase shift effect to set the exposure light transmittance of the halftone phase shift portion within the range of optimal transmittance (determined by each transfer pattern).+-.several %. In general, the optimal transmittance varies in a wide range, i.e., from 1% to 50%, according to transfer patterns, when the transmittance at the opening of the halftone phase shift layer is assumed to be 100%. That is, halftone phase shift masks having various levels of transmittance are demanded in order to deal with various patterns.
In actuality, the phase inversion function and transmittance control function are determined by the complex index of refraction (refractive index and extinction coefficient) of a substrate material and a material constituting a halftone phase shift film (a material constituting each layer, in the case of a multilayer film) and the film thickness. That is, a material usable as a halftone phase shift layer of a halftone phase shift photomask is required to have a transmittance for exposure light in the range of from 1% to 50% when the halftone phase shift film is formed on a substrate with the film thickness controlled so that the phase difference .phi. obtained by the above equation (1) falls within the range of n.pi..+-..pi./3 radians (n is an odd integer). As such material, films which are composed mainly of a chromium compound are known, as disclosed, for example, in Japanese Patent Application Laid-Open (KOKAI) No.5-127361 (1993).
Incidentally, the films composed mainly of a chromium compound are chromium oxide; chromium nitride, chromium oxide nitride, chromium oxide carbide, and chromium oxide carbide nitride, and the transmittances for exposure light of these films largely depend on the wavelength of exposure light. For example, FIG. 18 shows a spectral transmittance curve of a chromium oxide film formed on a synthetic quartz substrate by reactive sputtering in an oxygen atmosphere using a chromium target. In this case, the thickness of the chromium oxide film is about 50 nm. As will be clear from FIG. 18, the transmittance of the chromium oxide film rapidly falls off in the short wavelength region. Therefore, a halftone phase shift photomask that uses the chromium oxide film as a halftone phase shifter layer can be used for exposure at the g-line (436 nm) and i-line (365 nm) of a super-high pressure mercury lamp, but cannot be used for exposure using krypton fluoride excimer laser light (248 nm), whereby higher resolution can be realized, because the transmittance is excessively low. Similarly, the chromium nitride, chromium oxide nitride, chromium oxide carbide and chromium oxide carbide nitride films cannot be used for exposure using krypton fluoride excimer laser light and are therefore impossible to use for high-resolution lithography.